1. Field of the Invention
The present invention relates to a method of shaping and machining a disc wheel used for a vehicle and an equipment used for practicing said method.
2. Description of the Prior Art
A disc wheel used for a vehicle is fabricated by press fitting a disc in a rim and welding said rim and disc by spot welding or CO.sub.2 welding. The rim is provided with rim bead seat portions on both sides of the rim for holding a tire. The disc is provided with a hub hole at the center of the disc and it is fitted to the hub of a vehicle through the hub hole. The deviation of the rim bead portions radially from a circle concentric with the hub hole is required to be as small as possible. The smaller the amount of deviation of the rim bead seat portions from said concentric circle is, the more comfortable it will be to ride in the vehicle, while the larger the amount of the deviation is, the less comfortable it will be to drive because of occurrence of vibration during running of the vehicle.
According to the conventional way of forming and machining a disc wheel, for making the above-mentioned deviation small, first, a flat metallic plate is cut into band-shaped plates and said band-shaped plate is rounded to form a ring and both ends of thus rounded band-shaped plate are welded by flash butt welding to form a rough ring. Then the rough ring is passed through a forming machine having multi-stage forming rolls to form rim bead seat portions, being followed by correcting the shape of the rim by means of an expander or a shrinker. On the other hand, a disc is formed under a process different from said process of the rim and the hub hole is machined therein. And in the final step, the so-shaped rim and the so-formed and -machined disc are welded to obtain a disc wheel.
In such a conventional shaping method, however, there has been a problem such that because the expander and shrinker both comprise a split mold, the rim undergoes a local deformation at its portion positioned between the splits, and it is difficult to shape the circumference of the rim to be exactly round.
Moreover, since the shaping operation using a split mold is performed separately for each rim bead seat portion on each side of the rim, it has been difficult to shape both rim bead seat portions concentrically with each other.
Furthermore, because thus shaped rim and the disc thus machined to form a hub hole therein are welded to assembled disc wheel under the final manufacturing process, it has been difficult to align the axis of the circle of the rim bead seat portions and that of the hub hole completely with each other. And for the same reason, the dimension and the shape of the disc wheel are involved in both the errors caused by forming and shaping the rim and errors caused by forming and machining the disk and therefore, it has been difficult to attain a high geometrical accuracy.